Soils - erosion

Soil erosion is defined as the dislodgement of soil particles and their removal from their original position. It is a natural process and has been fundamental in the shaping of the Australian landscape over geological timescales. Places like the Kimberley Ranges are a dramatic testimony to the effects of erosion over long periods of time. However, human activities, and in particular agriculture, have greatly accelerated the rates of soil erosion.

Soil erosion is a major issue for Australian agriculture and catchment management. The rate of soil production in Australia is very low, and in many areas it is greatly exceeded by the rate of soil loss. Managing soil erosion can be difficult and costly.

This paper discusses the different types of water and wind erosion, the causes of accelerated erosion, and the extent and impacts of erosion on human and natural systems. It also touches on the management and prevention of erosion.

Processes and types of erosion

The susceptibility of soil to erosion and the rate at which it occurs are dependant upon a number of factors, including geology, climate, soil type, and density of vegetation.

Erosion can be broadly grouped into two forms, wind and water erosion. Accelerated water and wind erosion can occur where the soil surface is bare and exposed to intense rainfall and wind events. Human activities such as the clearance of vegetation, inappropriate cultivation practices and overgrazing can cause the disruption of the soil surface and increase its susceptibility erosion. The 2001 State of the Environment Report identified features that distinguish natural erosion processes from human-induced erosion.

Most wind erosion occurs in semi-arid and arid lands, and it has natural as well as human-induced components. The erosion and deposition of soil by wind has played a big role in the shaping of landforms across large parts of this continent, especially in the dune fields in the interior. However, over the past 150 years dune fields that were covered with natural vegetation have been cleared or grazed for agricultural purposes, leading to a greater rate of dust storms than would occur naturally.

Wind erosion results in the removal of large quantities of fertile topsoil containing organic matter, reducing the productive value of affected lands. Significant quantities of this dust are deposited off-shore, and on many occasions over the past century dust originating in Australia has been deposited as far away as New Zealand.

Dust storms are a useful indicator of wind erosion. The 1996 State of the Environment Report noted that since the 1970s there had actually been an overall decrease in the annual frequency of dust storms across. While some of this change is thought to be the result of increased rainfall spurring widespread vegetation recovery, it appears that the change is greater than can be explained by climatic conditions. This has been attributed to reduced rabbit populations through the release of the calicivirus, improvements in agricultural management practices resulting in increased vegetation cover and the spread of ‘woody weeds’.

Water erosion

There are several types of water erosion, the most important in Australia being sheet and rill erosion, tunnel erosion, gully erosion and stream channel erosion..

Sheet erosion occurs when a relatively uniform layer of topsoil is removed by raindrop splash or water run-off, and it can occur during severe storm events. It often affects areas where the soil is lacking in a protective vegetation cover. Rill erosion results from the concentration of surface water and runoff into deeper, faster moving channels, or rills, which follow low points through paddocks. The rills may be as deep as 30cm. Rill erosion can occur with sheet erosion, and it is commonly seen in paddocks that have been recently cultivated prior to intense rainfall.

Sheet and rill erosion result in the loss of topsoil and nutrients, with significant impacts on productivity. They can also have considerable off-site effects through the deposition of soil in streams, dams and reservoirs, reducing water quality and altering aquatic habitats.

Tunnel erosion occurs when water scouring or seeping through dispersive subsoils forms underground tunnels. These tunnels are often initiated by water accumulating and moving along cracks and channels, or into rabbit burrows or old tree root cavities. Sheet erosion can initiate tunnel erosion, by concentrating water in the subsoils or in low areas where there are rabbit burrows and tree root cavities.

When underground tunnels caused by this movement of water increase in size, parts of the tunnel roof may collapse, resulting in gullies and potholes. In fact, tunnel erosion is often not detected until tunnels begin to collapse and gullies are formed. The appearance of fine sediment downhill of a developing tunnel outlet point or hole, or even water seepage at the base of a slope, can indicate the early signs of tunnel erosion. Tunnel erosion can lead to a general loss of production, and can deposit infertile subsoils in lower, more productive areas. Its can also result in increased sediment loads in rivers and streams. In serious cases, tunnel erosion may , by causing gully erosion, restrict access across properties.

Gully erosion is one of the most visible and severe forms of water erosion. Gullies are steep-sided watercourses which experience ephemeral flows during heavy or extended rainfall. Advanced rill erosion may develop into gully erosion. Gully depth is, of course, limited by the depth of the underlying rock, so that most gullies are normally less than two metres deep. However, in deep alluvial and colluvial soils, they can reach depths of up to 15 metres.

The formation of erosion gullies can be triggered by various human activities, including cultivation and grazing leading to the loss of vegetation cover on soils susceptible to erosion. The concentration of run-off through furrows, contour banks, stock tracks, fences and roads can also lead to gully formation. Or it may be triggered when drainage lines are disrupted, through clearance of vegetation, diversion, or the construction of residential areas.

Gully erosion affects soil productivity, restricts land use, and can cause damage to fences, roads and even buildings. Its immediate impacts include the build-up of siltation along fence lines, waterways, road culverts, and in dams and water reservoirs. It can also result in water with high sediment loads reaching creeks and rivers, where nutrients and pesticides attached to soil particles can cause damage aquatic life.

Streambank erosion occurs when there is degradation of riparian vegetation. The 2001 National Land and Water Resources Audit estimates that it has been common for creeks and rivers cleared since European settlement to increase fourfold in depth and twofold in width. As in the case of gully erosion, streambank erosion can be a significant source of excess sediment in waterways, causing damage to floodplain land and infrastructure.

Extent of erosion

The maps shown in Figures 9.1 and 9.2 illustrate that large areas in Australia are highly susceptible to wind and water erosion. The 2001 National Land and Water Resources Audit estimated that almost 40% of the continent experiences low sheet and rill erosion (<0.5 t/ha/yr), 11 % experiences a high erosion risk and 50% experiences a medium erosion risk. The Audit found that the semi-arid woodlands and grazing lands in the Northern regions make the biggest total contribution to these kinds of soil erosion, although the highest rates of erosion were from tropical croplands.

With regard to gully and streambank erosion, the Audit estimated that around 30% of the Murray-Darling Basin was affected by moderate or high density erosion. Gully and streambank erosion were reported as being the dominant sources of sediment in waterways in southern Australia. The Audit found that 325 000 km of gullies across the area assessed had eroded about 4.4 billions tonnes of soil since European settlement. However, the Audit concluded that gully erosion in southern Australia had largely been stabilised, although gullies were still actively forming in northern Queensland and in some agricultural regions of Western Australia.

The Audit estimated that gully, streambank and sheetwash erosion deliver over 120 million tonnes of sediment to streams each year.

Management

Ultimately, vegetation cover is the most critical factor in the protection of soils from water and wind erosion. Any action that reduces the protective cover of vegetation increases the risk of soil loss. In the case of streambank erosion, access by stock to waterways can cause damage to riparian vegetation, increasing the risk of soil loss.

Vegetation should be encouraged in eroded areas, although it may be difficult to establish it in exposed, infertile soils. Indigenous plant species can be considered, but a number of exotic grasses and other plant species have been used with success in the control of erosion. In some cases, an initial application of fertilisers helps in the establishment of vegetation on degraded soils, and irrigation can also assist in the revegetation process. Stock should be excluded from eroded areas.

Other management techniques include contour banks, reduced or low tillage, low frequency of cropping, strip cropping and using windbreaks to reduce wind speeds.

The key to the management of erosion on grazing lands is control of grazing pressures. On all agricultural land, soil management to increase organic matter and to promote water infiltration and evapotranspiration by plants is vital to the management of soils degraded through erosion.

In the case of gully erosion, there are significant engineering approaches that can assist in the control of the problem, including gully reshaping and filling, and the construction of diversion banks and structures that minimise the impact of run-off.

The management of erosion can be very difficult and costly, and in many cases once erosion has started, it can only be minimised, but not stopped. The control of erosion over large areas of poor soils may be impractical.